Creating volumetric lighting (crepuscular rays) in Cycles
Update

As of Blender 2.70, cycles has had volumetric support. See this answer for details on how to set up crepuscular rays with the new volumetric features.



As mentioned by Wchargin, this will be possible once volumetrics is integrated.

However, there are some ways to fake this:


Using motion blur:


  Basically you use a plane that moves through the entire scene
  perpendicular to the camera, then turn on motion blur and increase the
  frame blending enough so that cycles is calculating the entire motion.
  The plane will catch the light and blend it through the whole scene
  giving the illusion of volume lighting. I used a mix of diffuse and
  translucent material for the plane but wouldn't be surprised if there
  was a better material method.

Using BI to render halo volumes and then use compositing nodes to combine with Cycles render